Line feed circuit with logic level controlled ringing

Abstract

A line-feed circuit which is controllable by logic level control signals from a local microprocessor. One of the logic level signals is a 20 Hz clock signal with a duty cycle of 20 ms high and 30 ms low. Another is an enable signal that allows the ringing driver either to be turned on (and which places the line-feed circuit into a disconnect or "open interval" state so as not to load the ringing signal) or off in which case the ringing driver is placed into a high impedance state so that it will not load signals appearing on the Ring conductor. This permits the ringing driver to be connected directly to the Ring conductor without the need for a series isolating element such as an electromechanical relay or comparable solid state device. The nature of the ringing driver allows the use of a simple ring trip detector comprised of a voltage comparator and duty cycle detector.

What is claimed is: 
     
       1. A ringing circuit for a telephone subscriber loop including a ring conductor comprising: a source of ringing clock signals having a predetermined duty cycle;   positive side ringing driver means for producing a linear positive ramp voltage having a controlled slew rate;   negative side ringing driver means fox producing a linear negative ramp voltage having a controlled slew rate;   means for applying said clock signal to said positive side driver means and for inverting and applying said clock signal to said negative side ringing driver means to operate said positive and negative side ringing driver means in push-pull in accordance with said duty cycle; and   means for alternately connecting each said ringing driver means to the ring conductor of said loop.   
     
     
       2. A ringing circuit for a telephone subscriber loop according to claim 1 wherein said positive and said negative ringing driver means includes: common capacitor means; and means for producing a fixed charging current for said capacitor means to produce said positive and said negative linear ramp voltage having said controlled slew rate.     
     
     
       3. A ringing circuit for a telephone subscriber loop according to claim 2 wherein said positive side ringing driver means pulls said one conductor toward ground potential at a predetermined slew rate and said negative side ringing driver pulls said one conductor toward a high negative potential at said predetermined slew rate to produce a trapezoidal ringing waveform on said one conductor. 
     
     
       4. A ringing circuit for a telephone subscriber loop according to claim 3 wherein said means for applying each said ramp voltage to said one conductor includes a mosfet output transistor respective to each said ringing driver means. 
     
     
       5. A ringing circuit for a telephone subscriber loop according to claim 4 including logic level signaling means for placing said mosfet output transistor in a high impedance state. 
     
     
       6. A ringing circuit for a telephone subscriber loop according to claim 5 wherein said logic level signaling means for placing said mosfet output transistor in said high impedance state includes an opto-isolator (U6A, U6B) for zeroing out the gate-to-source voltage of said mosfet irrespective of the voltage at said source. 
     
     
       7. A ringing circuit for a telephone subscriber loop according to claim 4 including means for limiting the peak current supplied to said one conductor of said loop by said respective mosfet output transistor. 
     
     
       8. A ringing circuit for a telephone subscriber loop according to claim 7 wherein said current limiter is triggered into operation by the off-hook condition of said loop. 
     
     
       9. A ringing circuit for a telephone subscriber loop according to claim 7 further including comparator means (C3D) for monitoring the voltage at the ringing driver output on a cycle by cycle basis to detect the ring trip condition. 
     
     
       10. A ringing circuit for a telephone subscriber loop according to claim 9 wherein said comparator means (C3D) produces an output signal (NRTA) which switches each time the output of the ringing driver crosses a predetermined negative voltage threshold. 
     
     
       11. A ringing circuit for a telephone subscriber loop according to claim 7 including means for monitoring the duty cycle of the voltage at said ringing driver output to detect the ring trip condition. 
     
     
       12. A ringing circuit for a telephone subscriber loop according to claim 11 wherein the duty cycle of said output signal is determined by the slicing level detected by said comparator.